This application claims the benefit of Japanese Application 2001-376,322, filed Dec. 10, 2001, the entirety of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a flexible printed wiring board comprising an electrically insulating substrate, a conductive pattern having a plurality of parallel electrically conductive strips serving as signal conductors arranged on a surface of the substrate, and an electrically insulating cover adhered to the surface of the substrate such that the conductive pattern is covered with the electrically insulating cover.
2. Related Art Statements
Flexible printed wiring boards of the kind mentioned above have been used in various applications. For instance, flexible printed wiring boards are provided in camera, duplicating machine, printer and hard disk devices. Nowadays, various types of flexible printed wiring boards have been widely used in cellular phones.
FIG. 1 is a cross sectional view showing a typical structure of a known flexible printed wiring board. On a surface of a flexible insulating substrate 1 is arranged a conductive pattern 2 including a plurality of electrically conductive strips 2-1, 2-2xe2x80x94which are arranged in parallel with each other with a given line width and spacing, the electrically conductive strips serving as signal conductors. An electrically insulating cover 3 is cemented to the substrate by means of an adhesive layer 4 such that the conductive pattern 2 is covered with the electrically insulating cover 3. Each of the conductive strips 2-1, 2-2xe2x80x94of the conductive pattern 2 has a width of, for instance 150-200 xcexcm and the conductive strips are arranged in parallel with each other with a space of, for instance 150-200 xcexcm. In a general manufacturing process of the conductive pattern 2, a copper film is first formed on the substrate by electrolytic plating with a thickness of, for instance 30-40 xcexcm and then the copper film is etched into a given pattern by a photolithography technique.
In the known flexible printed wiring board shown in FIG. 1, a symmetric structure is adopted in order to improve a flexibility, i.e. folding endurance. That is to say, the flexible printed wiring board is constructed such that a center of the conductive pattern 2 including a plurality of conductive strips 2-1, 2-2xe2x80x94is situated at a position which is substantially coincided with a geometrical center of the flexible printed wiring board viewed in a direction of a thickness of the flexible wiring board. It should be noted that the geometrical center may be considered as a neutral stress line of the flexible printed wiring board in a bent or folded condition. For example, when the electrically insulating substrate 1 is made of polyimide and has a thickness of 50 xcexcm, a thickness of the electrically insulating cover 3 is set to 25 xcexcm and a thickness of the adhesive layer 4 is also set to 25 xcexcmm. In many applications, very severe conditions have been imposed upon the folding endurance. For instance, in a flexible printed wiring board for use in a folding type cellular phone, break down of the conductive strip should not occur even after 50,000 times of bending, preferably not even after 100,000 times of bending, and much more preferably not even after 200,000 times of bending. However, the known flexible printed wiring board illustrated in FIG. 1 could not sufficiently satisfy such a severe condition.
When a flexible printed wiring board is used in a communicating machine such as cellular phone, it is required to transmit a higher frequency signal through the flexible printed wiring board in accordance with an increase in a communication speed. In such a high frequency application, in order to transmit a high frequency signal efficiently, accurate impedance matching is required. To this end, successive conductive strips of a conductive pattern are used alternately as signal lines and ground lines. However, in the known flexible printed wiring board shown in FIG. 1, impedance matching could not be attained accurately. In order to effect the impedance matching accurately, the spacing between successive conductive strips of the conductive pattern 2 has to be controlled precisely. However, this control is very difficult. Particularly, in recent cellular phones, it is required to set an impedance within a range of 50xcexa9xc2x110%, but the impedance of the flexible printed wiring board could not be fit within such a narrow range.
FIG. 2 is a cross sectional view depicting another known flexible printed wiring board, in which portions similar to those shown in FIG. 1 are denoted by the same reference numerals used in FIG. 1. Also in this known flexible printed wiring board, conductive strips 2-1, 2-2xe2x80x94having a given line width of a conductive pattern 2 are arranged in parallel with each other at a given spacing on a surface of a flexible electrically insulating substrate 1, and an electrically insulating cover 3 is secured to the surface of the substrate 1 by means of an adhesive layer 4 such that the conductive pattern 2 is covered with the electrically insulating cover 3. The conductive strips 2-1, 2-2xe2x80x94alternately serve assignal lines and ground lines. The conductive strips 2-1, 2-3, 2-5xe2x80x94serving as signal lines are independent form each other, but the conductive strips 2-2, 2-4, 2-6xe2x80x94serving as ground lines are commonly connected to a ground sheet or earth sheet 6 by means of vias 5 formed by embedding an electrically conductive material in via holes formed in the electrically insulating substrate 1. The ground sheet 6 is covered with an electrically insulating cover 7. This type flexible printed wiring board is disclosed in a Japanese Patent Specification No. 2,549,213.
In the conventional flexible printed wiring board illustrated in FIG. 2, since the ground sheet 6 is arranged below the signal lines, precise impedance matching can be performed. However, the structure is rather complicated and the manufacturing process is also cumbersome, and the cost is liable to be high. Moreover, a decrease in life time due to repeated bending is large, and the flexible printed wiring board could not be used for applications in which a folding endurance should be maintained after more than several tens of thousands times of bending.
In the known flexible printed wiring boards illustrated in FIGS. 1 and 2, the conductive strips 2-1, 2-2, 2-3xe2x80x94arranged on the same plane are used alternately as signal lines and ground lines, and thus a utilization efficiency of a surface area of the flexible printed wiring board for the signal transmission is low. Due to the high performance of electronic devices, the amount of signal to be transmitted through the flexible printed wiring board becomes larger and larger. However, in the known flexible printed wiring boards shown in FIGS. 1 and 2, if the number of conductive strips is increased, the width of the wiring boards has to be larger and a high density package could not be attained. Alternatively, if the number of flexible printed wiring boards to be used is increased, the whole structure becomes large and complicated and the cost is increased. Moreover, the folding endurance becomes lower and the flexible printed wiring boards could not be used in applications in which the folding endurance should not be lost after several tens of thousands times of bending.
The present invention has for its object to provide a novel and useful flexible printed wiring board, in which the impedance matching can be attained easily and precisely without decreasing the amount of signal to be transmitted and without lowering the folding endurance, while the flexible printed wiring board can be manufactured in a simple and less expensive manner.
It is another object of the invention to provide a flexible printed wiring board in which impedance matching can be attained easily and precisely without deceasing the amount of signal to be transmitted and having a folding endurance that is higher than the known flexible printed wiring boards.
According to one aspect of the present invention, a flexible printed wiring board is provided, including a flexible electrically insulating substrate having first and second major surfaces, a first electrically conductive pattern including a plurality of conductive strips arranged in parallel with each other on the first major surface of the electrically insulating substrate, a second electrically conductive pattern including a plurality of conductive strips arranged in parallel with each other on the second major surface of the electrically insulating substrate, a first flexible electrically insulating cover provided on the first major surface of the electrically insulating substrate such that said first conductive pattern is covered with the first flexible electrically insulating cover, and a second flexible electrically insulating cover provided on the second major surface of the electrically insulating substrate such that the second conductive pattern is covered with the second flexible electrically insulating cover.
In the flexible printed wiring board according to the invention, a part or all of conductive strips of the first and second conductive patterns may be used as signal lines and the remaining one or more conductive strips are used as ground lines. Therefore, impedance matching for a high frequency signal can be attained easily and a high frequency signal can be transmitted efficiently with a small loss. Furthermore, the first and second conductive patterns are separated in the thickness direction of the flexible printed wiring board, and thus, the amount of signal to be transmitted can be increased in comparison with the conventional flexible printed wring board illustrated in FIG. 2. Moreover, it is not necessary to provide the ground sheet having a large surface area that is provided in the known flexible printed wiring board shown in FIG. 2, and therefore the folding endurance is not substantially decreased.
In a preferred embodiment of the flexible printed wiring board according to the invention, the first and second conductive patterns are provided at such a position viewed in the thickness direction of the wiring board such that the first and second conductive patterns are situated symmetrically on respective sides of a geometrical center line or neutral stress line. In such a symmetrical structure, the folding endurance can be improved in applications in which the flexible printed wiring board in a flat condition is bent both directions. Furthermore, the flexible printed wiring board having a symmetrical structure can be manufactured much more easily.
In another preferred embodiment of the flexible printed wiring board according to the invention, the first and second conductive patterns are arranged asymmetrically with respect to a geometrical center line or neutral stress line toward an inner side of the wiring board under a bent condition when viewed in the thickness direction of the flexible printed wiring board. In this case, it is particularly preferable that both the first and second conductive patterns are situated on the inner side of the geometrical center line or neutral stress line. In such an asymmetrical structure, the portions of the conductive pattern positioned far from the geometrical center line or neutral stress line are subjected to a larger compressive stress upon bending, but it has been confirmed experimentally that folding endurance is less affected by compressive stress.
According to a second aspect of the present invention, a flexible printed wiring board is provided, including first, second and third flexible electrically insulating substrates each having first and second major surfaces, a first electrically conductive pattern including a plurality of conductive strips serving as ground lines arranged in parallel with each other on a first major surface of the first electrically insulating substrate, a second electrically conductive pattern including a plurality of conductive strips serving as signal lines arranged in parallel with each other on a first major surface of the second electrically insulating substrate, a third electrically conductive pattern including a plurality of conductive strips serving as ground lines arranged in parallel with each other on a first major surface of the third electrically insulating substrate, a first adhesive layer cementing the first major surface of the first electrically insulating substrate to a second surface of the second electrically insulating substrate, a second adhesive layer cementing the first major surface of the second electrically insulating substrate to a second surface of the third electrically insulating substrate; a flexible electrically insulating cover, and a third adhesive layer cementing the flexible electrically insulating cover to the first major surface of the third electrically insulating substrate.
In the above-mentioned flexible printed wiring board according to the second aspect of the present invention, the second conductive pattern serving as signal lines is sandwiched between the first and third conductive patterns serving as ground lines, and therefore impedance matching can be attained much more easily. In this case, all the conductive strips of the first, second and third conductive patterns may be aligned in the thickness direction of the wiring board, but it is preferable that the middle second conductive pattern is deviated laterally by a half pitch. Then, undesired fluctuation of impedance can be effectively suppressed.
The flexible printed wiring board according to the invention may have a symmetrical structure, but it is particularly preferable that the first, second and third conductive patterns are arranged asymmetrically with respect to a geometrical center line or neutral stress line of the flexible printed wiring board under a bent condition viewed in the thickness direction of the flexible printed wiring board. That is to say, the first, second and third conductive patterns are deviated or shifted toward an inner side of a geometrical center line or neutral stress line of the flexible printed wiring board under a folded condition. Such an asymmetrical structure can further improve a folding endurance of the conductive patterns.
In the flexible printed wiring board according to the invention, the electrically insulating substrate and flexible electrically insulating cover may be advantageously made of polyimide having a superior electrical insulating property as well as a superior folding endurance, and the conductive pattern may be made of a metal having an extremely high folding endurance such as copper, beryllium copper alloy and phosphor bronze. The conductive pattern may be made of any other electrically conductive material depending upon applications. In any case, it is preferable that the conductive pattern is made of a material having a conductivity which is not less than 1% of the conductivity of pure copper, preferably not less than 5% of the conductivity of pure copper, and more particularly not less than 10% of the conductivity of a pure copper. Furthermore, according to the invention, it is preferable that the impedance of the flexible printed wiring board is set to a value within 50xcexa9xc2x120%, preferably 50xcexa9xc2x110%, and more particularly 50xcexa9xc2x15%.